In a conventional color image forming apparatus, toner images of component colors, such as yellow, magenta, cyan, and black, are formed on a photosensitive drum, transferred and stacked on recording paper, and then fixed to form a color image on the recording paper. If the toner images of component colors are not stacked appropriately, colors are not reproduced as intended due to color displacement. The result is a color image which looks ugly.
To address this problem, a color displacement determining pattern for determining how much the colors are displaced is formed on the recording paper or a carrier for the recording paper and then inspected to measure a color displacement. The inspection, in its most primitive form, was done visually by, for example, examining an image formed on recording paper using a highly efficient magnifying glass with a scale. Alternatively, the color displacement was measured mechanically using a special measurement instrument. Drawbacks of these methods include the need for dedicated equipment for the inspection and a prolonged process to perform the measurement and accompanying cost increases in compensating for color displacements.
Another possible alternative is to provide means to check for a color displacement inside the image forming apparatus. However, when this is the case, image reading means is necessary which reads a color displacement determining pattern on the recording paper. If the image reading means is disposed solely for the inspecting purpose, the overall cost of the image forming apparatus increases; therefore, a prerequisite is that a reading device for reading an original is provided in the image forming apparatus and the device can play an additional role in the inspection. Making use of the reading device, the color displacement determining pattern is read, the color displacement is measured, and the resist of the component color images is adjusted according to the color displacement.
For example, Japanese Laid-Open Patent Application No. 3-139961/1991 (Tokukaihei 3-139961; published on Jun. 14, 1991) discloses a technique addressing the problem of inaccuracy of visual inspection and troublesome work involved. According to the technique, a test chart is formed on recording paper by two or more image forming heads and then read by image reading means. The color displacement is measured based on the reading of the test chart, and the recording timings of the image forming heads are corrected in accordance with the displacement.
However, if, as in this conventional technique, a test chart formed on recording paper is read by image reading means, since the operator places the recording paper in the image reading means, the measurement may vary depending on how he/she places the recording paper. The position of the recording paper should be carefully adjusted in the image reading means. Trouble arises where it is difficult to determine whether the recording paper is appropriately positioned and to adjust the position of the recording paper appropriately. Provided that the recording paper is ever positioned appropriately, it is still extremely difficult to position the test chart appropriately and read it using image reading means, because the recording paper may tilt while manipulating a press on the recording paper or the test chart may have been displaced on the recording paper in the first place.
Japanese Laid-Open Patent Application No. 6-95474/1994 (Tokukaihei 6-95474; published on Apr. 8, 1994) discloses a technique addressing these inconveniences. According to the technique, a pattern is produced in which there are a plurality of line elements running parallel or perpendicular to the transport direction of the recording paper and at least one line element crossing the plurality of line elements at right angles. The produced pattern is recorded on the recording paper and read using image reading means to measure displacements of the component color images and correct the resist of the component color images. Thus, the technique can solve problems that the pattern is read diagonally and resultant reading of its position is inaccurate, due to vertical or horizontal shaking or other reasons.
The use of these conventional techniques still comes short of producing satisfactory results: measurements of displacements of the component color images are adversely affected by where the pattern is positioned on the recording paper, since each piece of image forming means for forming component color images has its own irregularity in operation. The result is inaccurate correction of the resist of the component color images. In some cases, repeated measurements and corrections are necessary.